Shear force measurements on low- and high-stiffness posterior fusion devices

Biomechanical Comparison of Robotically Applied Pure Moment, Ideal Follower Load, and Novel Trunk Weight Loading Protocols on L4-L5 Cadaveric Segments during Flexion-Extension

BACKGROUND

Extremely few in-vitro biomechanical studies have incorporated shear loads leaving a gap for investigation, especially when applied in combination with compression and bending under dynamic conditions. The objective of this study was to biomechanically compare sagittal plane application of two standard protocols, pure moment (PM) and follower load (FL), with a novel trunk weight (TW) loading protocol designed to induce shear in combination with compression and dynamic bending in a neutrally potted human cadaveric L4-L5 motion segment unit (MSU) model. A secondary objective and novelty of the current study was the application of all three protocols within the same testing system serving to reduce artifacts due to testing system variability.

METHODS

Six L4-L5 segments were tested in a Cartesian load controlled system in flexion-extension to 8Nm under PM, simulated ideal 400N FL, and vertically oriented 400N TW loading protocols. Comparison metrics used were rotational range of motion (RROM), flexibility, neutral zone (NZ) range of motion, and L4 vertebral body displacements.

RESULTS

Significant differences in vertebral body translations were observed with different initial force applications but not with subsequent bending moment application. Significant reductions were observed in combined flexion-extension RROM, in flexibility during extension, and in NZ region flexibility with the TW loading protocol as compared to PM loading. Neutral zone ranges of motion were not different between all protocols.

CONCLUSIONS

The combined compression and shear forces applied across the spinal joint in the trunk weight protocol may have a small but significantly increased stabilizing effect on segment flexibility and kinematics during sagittal plane flexion and extension.

Characterization of the behavior of a novel low-stiffness posterior spinal implant under anterior shear loading on a degenerative spinal model

PURPOSE

Dynamic implants have been developed to address potential adjacent level effects due to rigid instrumentation. Rates of revision surgeries may be reduced by using improved implants in the primary surgery. Prior to clinical use, implants should be rigorously tested ex vivo. The objective of our study was to characterize the load-sharing and kinematic behavior of a novel low-stiffness spinal implant.

METHODS

A human cadaveric model of degenerative spondylolisthesis was tested in shear. Lumbar functional spinal units (N = 15) were tested under a static 300 N axial compression force and a cyclic anterior shear force (5-250 N). Translation was tracked with a motion capture system. A novel implant was compared to three standard implants with shear stiffness ranging from low to high. All implants were instrumented with strain gauges to measure the supported shear force. Each implant was affixed to each specimen, and the specimens were tested intact and in two progressively destabilized states.

RESULTS

Specimen condition and implant type affected implant load-sharing and specimen translation (p < 0.0001). Implant load-sharing increased across all degeneration-simulating specimen conditions and decreased across the three standard implants (high- to low-stiffness). Translation increased with the three standard implants (trend). The novel implant behaved similarly to the medium-stiffness implant (p > 0.2).

CONCLUSIONS

The novel implant behaved similarly to the medium-stiffness implant in both load-sharing and translation despite having a different design and stiffness. Complex implant design and specimen-implant interaction necessitate pre-clinical testing of novel implants. Further in vitro testing in axial rotation and flexion-extension is recommended as they are highly relevant loading directions for non-rigid implants.

Biomechanical stability of lateral interbody implants and supplemental fixation in a cadaveric degenerative spondylolisthesis model

STUDY DESIGN

In vitro cadaveric biomechanical study of lateral interbody cages and supplemental fixation in a degenerative spondylolisthesis (DS) model.

OBJECTIVE

To investigate changes in shear and flexion-extension stability of lateral interbody fusion constructs.

SUMMARY OF BACKGROUND DATA

Instability associated with DS may increase postoperative treatment complications. Several groups have investigated DS in cadaveric spines. Extreme lateral interbody fusion (XLIF) cages with supplemental fixation have not previously been examined using a DS model.

METHODS

Seven human cadaveric L4-L5 motion segments were evaluated using flexion-extension moments to ±7.5 N·m and anterior-posterior (A-P) shear loading of 150 N with a static axial compressive load of 300 N. Conditions were: (1) intact segment, (2) DS simulation with facet resection and lateral discectomy, (3) standalone XLIF cage, (4) XLIF cage with (1) lateral plate, (2) lateral plate and unilateral pedicle screws contralateral to the plate (PS), (3) unilateral PS, (4) bilateral PS, (5) spinous process plate, and (6) lateral plate and spinous process plate. Flexion-extension range of motion (ROM) data were compared between conditions and with results from a previous study without DS simulation. A-P shear displacements were compared between conditions.

RESULTS

Flexion-extension ROM after DS destabilization increased significantly by 181% of intact ROM. With the XLIF cage alone, ROM decreased to 77% of intact. All conditions were less stable than corresponding conditions with intact posterior elements except those including the spinous process plate. Under shear loading, A-P displacement with the XLIF cage alone increased by 2.2 times intact. Bilateral PS provided the largest reduction of A-P displacement, whereas the spinous process plate alone provided the least.

CONCLUSION

This is the first in vitro shear load testing of XLIF cages with supplemental fixation in a cadaveric DS model. The variability in sagittal plane construct stability, including significantly increased flexion-extension ROM found with most fixation conditions including bilateral PS may explain some clinical treatment complications in DS with residual instability.

LEVEL OF EVIDENCE

N/A.